Cambridge-Washington FT3/FT3C
Lightguide (fiber-optic) Route Map

The Cambridge-Washington system was AT&T's first fiber-optic route, placed in service around 1983-84.
The technology was initially designated as type FT3 and had a capacity of 45 Mb/sec, but by 1984 had been upgraded to FT3C, carrying 90 Mb/sec.

"Lightguide" was AT&T's original name for optical fiber transmission; it was derived from "waveguide", the name for the hollow pipes used to carry microwave signals.